In modern mobile communications, direct conversion receivers are widely used for wireless communications due to their lower complexity and lower power consumption. In a direct conversion receiver, a received radio frequency signal is down-converted to baseband In-phase (I) and Quadrature-phase (Q) signal components to estimate the phase and amplitude of the received radio frequency signal. In an ideal case, the phase difference between the baseband I and Q signal components should be 90 degrees in order to correctly estimate the phase and amplitude of the received radio frequency signal. However, due to temperature dependencies and production imperfections in the direct conversion receiver, the phase difference between the baseband I and Q signal components may not be exactly 90 degrees causing an amplitude and phase imbalance to occur between the baseband I and Q signal components. The amplitude imbalance refers to deviation/error between amplitudes of the baseband I and Q signal components. Similarly, the phase imbalance refers to deviation/error between phase of the baseband I and Q signal components. This amplitude and phase imbalance between the baseband I and Q signal components degrades the reception performance of the direct conversion receiver resulting in audible distortion.
The above mentioned problem of the amplitude and phase imbalance between the baseband I and Q signal components is well known in the art and several approaches exist for estimating and correcting this amplitude and phase imbalance. However, the existing methods make use of either training signals/tone or preamble signal, and such training sequences are difficult to build requiring significant software and hardware modifications. In fact, two way radios that support analog mode do not use preamble signals making it very difficult to estimate and correct the amplitude and phase imbalance errors.
Some of the existing methods estimate the amplitude and phase imbalance errors between the baseband I and Q signal components using a High Order Statics (HOS) technique. The HOS technique makes assumptions about the signal sources thus making it unsuitable for constant envelope FM based 2-way radio. Moreover, HOS techniques are computationally intensive making it infeasible for implementation on an embedded platform.
Therefore, there is a need for a solution that does not require use of any training sequences, preamble signals or HOS technique for estimating and correcting the amplitude and phase imbalance between the baseband I and Q signal components.
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